I'm pretty sure that Ommegang uses their own yeast strain, not the Duvel strain. From what I understand, it's not available to homebrewers, but they use the same house strain for all their brews so you could culture it from any of their beers.

And that would also explain why the recipe you have doesn't call for a Saison strain. From what I understand, Ommegang is just using their house strain for Hennepin.

I've grown up a pitchable amount from Ommegang bottle dregs before. It's actually pretty easy as the dregs seem to be more viable than some of the Belgian beers that have traveled further and been stored longer.

It's interesting to hear that they use spices to mimic different yeasts, as I did not get much character from their yeast. I pitched it into a straight pils wort, so it was a pretty simple beer but not remarkable in any fashion. It attenuated like mad.